In recent years, water absorbable hydrogels have been used not only as the materials for paper diapers and tampons, but also as liquid absorbents for use in medical care, construction, civil engineering, building, etc. Moreover, the water absorbable hydrogels can also be used as texture enhancers, fresh-keeping agents for foods, important basal material for greening engineering in the fields of horticulture and other agricultural applications.
Conventional methods for preparing water absorbable hydrogels use starches and celluloses cross-linked with acrylnitrile to form acrylate-based water absorbable hydrogels. Although such acrylate-based hydrogels are cheap, they can be partially decomposed by microorganisms in the soil and encounter difficulties in waste treatment and problems of toxicity toward human bodies. It is believed that imparting water absorbable hydrogels with good biodegradability will resolve the problem regarding the waste treatment. Therefore, there is a great demand for biodegradable, water absorbable hydrogels in view of the increasing environmental concerns.
In order to overcome the aforementioned problem, conventional techniques involve using biodegradable starch-based, hyaluronic acid-based, or polyamino acid-based, or materials as the starting materials for the preparation of biodegradable, water absorbable hydrogels. The methods for the preparation of polyamino acid-based, cross-linked products have been disclosed in the prior art, such as JP 6-322358, JP 7-224163, JP 7-309943, JP 7-300563, JP 10-298282, and JP 11-343339. For example, JP 6-322358 indicates that a solution of γ-PGA can be cross-linked via an electronic polymerization mechanism by using strong gamma-irradiation, so as to form γ-PGA based, cross-linked product. However, the equipments for producing γ-PGA based, cross-linked products through irradiation is very complicated and restricted, such that the production procedure is difficult and inconvenient. JP 11-343339 discloses another method for preparing cross-linked γ-PGA product, comprising isolating a high concentration of γ-PGA from a culture broth, and using the isolated γ-PGA as the starting material for the cross-linking reaction with a di-epoxy compound to obtain a biodegradable, water absorbable hydrogel. Nonetheless, such method not only has the drawback associated with the requirement of a high concentration of γ-PGA, obtained through the procedures including a cell separation and extraction of γ-PGA from a microbial culture broth through a refining step, but also requires particular operational equipments to improve the solubility of γ-PGA and the di-epoxy compound in a solvent. Moreover, the above method uses γ-laser irradiation to complete the cross-linking reaction between γ-PGA and the di-epoxy compound. Obviously, the preparation technology disclosed in JP 11-343339 also causes the problems regarding an increase in cost and inconvenience in preparation procedure.
Moreover, JP 5-301904 discloses polysaccharides produced from Alcaligenes letus B16. U.S. Pat. No. 4,772,419 also discloses a method for the preparation of cross-linked polysaccharide products.
Conventional methods for manufacturing cross-linked γ-PGA products require complicated processing procedures, such as the control and operation of complicated irradiation equipments and the separation and refining steps. Also, γ-PGA hydrogel products obtained by known technology are relatively unstable and easily decomposed in few days (3 to 5 days) after fully swelling in water at room temperature. Surprisingly, the inventors of the present application have found that good biodegradable, high water absorbable γ-PGA hydrogels having up to 5,000 times water absorption capacity, improved firmness, and long-term stability after full swelling in water or an aqueous medium can be directly, simply, and successfully prepared.